Fire resistant, all dielectric fiber optic cable

Abstract

Disclosed is a fire resistant, all dielectric fiber optic cable with high fiber count. The cable comprises a core including a central strength member and buffer tubes containing fibers, arranged around the central strength member. A first mica layer is arranged around the core. A glass yarn layer surrounds the first mica layer and is in direct contact therewith. A inner sheath surrounds the glass yarn layer. A second mica layer surrounds the inner sheath. An outer sheath surrounds the second mica layer. The buffer tubes contain a water-blocking filling material comprising a silicone gel, wherein said silicone gel has a drop point of at least 200° C.

Claims

1. Fiber optic cable comprising: a core comprising: a central strength member, and a plurality of buffer tubes arranged around the central strength member, each of the plurality of buffer tubes containing a plurality of optical fibers; a first mica layer arranged around the core; a glass yarn layer surrounding and in direct contact with the first mica layer; an inner sheath surrounding the glass yarn layer; a second mica layer surrounding the inner sheath; and an outer sheath surrounding the second mica layer, wherein each of the plurality of buffer tubes comprise a water-blocking filling material comprising a silicone gel, wherein the silicone gel has a drop point of at least 200° C., wherein the fiber optic cable is a fire resistant all dielectric fiber optic cable.

2. The fiber optic cable of claim 1, wherein the plurality of optical fibers comprises at least 24 optical fibers.

3. The fiber optic cable of claim 1, wherein the central strength member comprises a hydroxide-containing flame retardant polymeric material.

4. The fiber optic cable of claim 3, wherein the plurality of optical fibers comprises at least 72 optical fibers.

5. The fiber optic cable of claim 3, wherein the central strength member comprises a body and the hydroxide-containing flame retardant polymeric material is in form of a layer on the outer surface of the body.

6. The fiber optic cable of claim 3, wherein the hydroxide-containing flame retardant polymeric material of the central strength member is a LSoH material.

7. The fiber optic cable of claim 3, wherein the hydroxide-containing flame retardant polymeric material of the central strength member has a limiting oxygen index (LOI)≤35%.

8. The fiber optic cable of claim 1, wherein the silicone gel has a drop point of ≥250° C.

9. The fiber optic cable of claim 1, wherein a water swellable material is interposed between the core and the first mica layer and/or between the glass yarn layer and the inner sheath and/or between the inner sheath and the second mica layer.

10. The fiber optic cable of claim 1, wherein the inner sheath and/or the outer sheath are made of a flame retardant LSoH polymer-based material.

11. The fiber optic cable of claim 10, wherein the flame retardant LSoH polymer-based material has a limiting oxygen index (LOI)≥30%.

12. The fiber optic cable of claim 10, wherein the flame retardant LSoH polymer-based material has a limiting oxygen index (LOI)≤70%.

13. The fiber optic cable of claim 10, wherein the inner sheath and the outer sheath are made of the same LSoH flame-retardant polymer-based material.

14. The fiber optic cable of claim 1, wherein the inner sheath and the outer sheath are made of a flame retardant LSoH polymer-based material, and wherein the flame-retardant LSoH polymer-based material of the inner sheath has a LOI higher than the LOI of the outer sheath material.

15. The fiber optic cable of claim 1, wherein at least one the first mica layer and of the second mica layer comprises two mica tapes wound in the same direction.

16. The fiber optic cable of claim 1, wherein a water swellable material is interposed between the core and the first mica layer, and between the glass yarn layer and the inner sheath, and between the inner sheath and the second mica layer.

17. A fiber optic cable comprising: a core comprising: a central strength member, and a plurality of buffer tubes arranged around the central strength member, each of the plurality of buffer tubes containing a plurality of optical fibers; a first mica layer arranged around the core; a glass yarn layer surrounding and in direct contact with the first mica layer; an inner sheath surrounding the glass yarn layer; a second mica layer surrounding the inner sheath; an outer sheath surrounding the second mica layer; and a water swellable material interposed between the core and the first mica layer, or between the glass yarn layer and the inner sheath, or between the inner sheath and the second mica layer, wherein each of the plurality of buffer tubes comprise a water-blocking filling material comprising a silicone gel, wherein the silicone gel has a drop point of at least 200° C., wherein the fiber optic cable is a first resistant all dielectric fiber optic cable.

18. The fiber optic cable of claim 17, wherein the central strength member comprises a hydroxide-containing flame retardant polymeric material, and wherein the inner sheath and the outer sheath is made of a flame retardant LSoH polymer-based material.

19. A fiber optic cable comprising: a core comprising: a central strength member, and a plurality of buffer tubes arranged around the central strength member, each of the plurality of buffer tubes containing a plurality of optical fibers; a first mica layer arranged around the core; a glass yarn layer surrounding and in direct contact with the first mica layer; an inner sheath surrounding the glass yarn layer; a second mica layer surrounding the inner sheath; an outer sheath surrounding the second mica layer, wherein the inner sheath or the outer sheath is made of a flame retardant LSoH polymer-based material; and wherein each of the plurality of buffer tubes comprise a water-blocking filling material comprising a silicone gel, wherein the silicone gel has a drop point of at least 200° C., wherein the fiber optic cable is a first resistant all dielectric fiber optic cable.

20. The fiber optic cable of claim 19, further comprising a water swellable material interposed between the core and the first mica layer, and between the glass yarn layer and the inner sheath, and between the inner sheath and the second mica layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present disclosure will become fully clear after reading the following detailed description, given by way of example and not of limitation, with reference to the attached FIG. 1 which is a cross-section of a fire resistant, all dielectric fiber optic cable according an embodiment of the present disclosure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(2) In FIG. 1, the all dielectric fiber optic cable is indicated by reference number 1.

(3) Cable 1 comprises a core 1a. The core 1a, in turn, comprises a central strength member 2 and a plurality of buffer tubes 4, each comprising a plurality of optical fibers 5.

(4) The central strength member 2 is an elongated element and it can have a circular or substantially circular cross-section. In the present embodiment, the central strength member comprises a body 2a of reinforced dielectric material, for example glass reinforced plastic (GRP), fiber reinforced plastic (FRP) or any other similar material.

(5) The body 2a is covered by a coating 2b made of hydroxide-containing flame retardant LSoH polymeric material. The material of the coating 2b contains magnesium hydroxide in an amount of about 130 phr. This material can have a LOI of 28%.

(6) A number of buffer tubes 4 are arranged radially outer with respect to the central strength member 2. In an embodiment, the buffer tubes 4 are stranded around the central strength member 2 in S-Z configuration.

(7) In the embodiment of FIG. 1, eight buffer tubes 4 are provided around the central strength member 2. However, there could be more or less tubes in other embodiments.

(8) The buffer tubes 4 can be made of any suitable polymeric material, for example polybutylene terephthalate (PBT). In an embodiment, the buffer tubes can be made of a hydroxide-containing flame retardant LSoH polymeric material.

(9) Each buffer tube 4 contains a plurality of optical fibers. In an embodiment, each buffer tube 4 contains 12 optical fibers.

(10) Each buffer tube 4 may contain water-blocking filling material 6 comprising a silicone gel with a drop point of at least 200° C.

(11) For example, materials suitable as water-blocking filling for the all dielectric cable of the present disclosure are polyorganosiloxane marketed as Rhodorsil® by Rhodia Siliconi Italia S.p.A., Italy.

(12) It should be remarked that each single buffer tube 4 is not individually protected by fire resistant materials, for example mica tapes.

(13) The core 1a comprising buffer tubes 4 and the central strength member 2 is wrapped by a first mica layer 7.

(14) In an embodiment, the first mica layer 7 comprises two mica tapes. Mica, for example in form of flakes, may be bonded to a backing layer using a binding agent, such as silicone resin or elastomer, acrylic resin and/or epoxy resin. The backing layer may be formed of a supporting fabric, such as woven glass and/or glass cloth.

(15) In an embodiment, each mica tape is wound with an overlapping. The overlapping can be higher than 40%, for example of 50%.

(16) In radial external position and in direct contact with the first mica layer 7 a layer of glass yarns 8 is provided.

(17) The layer of glass yarns 8 and the first mica layer 7 act as fire barrier. The fire barrier layer has mainly the function of avoiding direct contact of the inner core with the flames which surround the cable in case of fire.

(18) A water barrier (not illustrated), which is dielectric and made of water swellable material (for example, supported by a tape) may surround the core 1a in a radial position internal with respect to the first mica layer 7 or over the glass yarns 8.

(19) In radial external position and in direct contact with the layer of glass yarns 8, an inner sheath 9 is provided. The inner sheath 9 can be extruded directly on the layer of glass yarns 8. In an alternative embodiment, water swellable material in form of one or more water swellable tapes can be interposed between the layer of glass yarns 8 and the inner outer sheath 9.

(20) The inner sheath 9 can have a thickness between 1 and 3 mm. In one embodiment, such thickness is of 2.5 mm.

(21) The inner sheath 9 is made of a flame retardant LSoH polymer material. The material of the inner sheath 9 contains magnesium hydroxide in an amount of about 200 phr. This material can have a LOI of 37%.

(22) In radially outer position with respect to the inner sheath 9, a second mica layer 10 is provided. The second mica layer 10 may comprise two mica tapes similar to those used for the first mica layer 7. In an embodiment, each mica tape of the second mica layer 10 is wound with an overlapping. The overlapping can be higher than 40%, for example of 50%.

(23) In a radially outer position to and in direct contact with the second mica layer 10, an outer sheath 11 is provided. The outer sheath 11 can be extruded directly on the second mica layer 10. In an alternative embodiment, a polyester tape can be interposed between the second mica layer and the outer sheath.

(24) The outer sheath 11 can have a minimum thickness between 1 and 3 mm. In one embodiment, such thickness is of 2.5 mm.

(25) Outer sheath 11 is made of a flame retardant LSoH polymeric material. The material of the outer sheath 11 contains magnesium hydroxide in an amount of about 200 phr. This material can have a LOI of 37%.

Example

(26) The Applicant has conducted fire-resistance tests on different cables.

(27) All of the tested cables have a structure analogous to that of cable 1 of FIG. 1 and contained 144 fibers in 12 buffer tubes, 12 fibers each. The buffer tubes of Cable 1 (comparative) were filled with a severely hydrotreated and hydrocracked base mineral oil having a drop point lower than 200° C. (about 180° C.), while the buffer tubes of Cable 2 (according to the present disclosure) were filled with a polydimethylsiloxane gel having a drop point greater than 250° C.

(28) Cable 1 (comparative) and Cable 2 had a central strength member made of GRP hydroxy-containing flame retardant polymeric material having a LOI of 28%.

(29) The inner and outer sheath of Cable 1 and Cable 2 were made of the same flame retardant LSoH polymer-based material containing magnesium hydroxide and having a LOI of about 37%.

(30) Cable 1 (comparative) failed the fire resistance test performed on these cables according to IEC 60331-25 (1999). After 90 minutes at 750° C., a 2.54 dB signal attenuation was detected. After a cooling, a 5.58 dB signal attenuation was observed and cracks of the optical fibers were detected. Cable 2 according to the present disclosure passed the fire resistance test performed according to IEC 60331-25 (1999). After 90 minutes at 750° C., a 1.34 dB signal attenuation was detected. After a cooling of 15 minutes, a 1.51 dB signal attenuation was detected, and the optical fibers were found to be fully functioning (not breakage, no signal disconnection).

(31) Cable 2 according to the present disclosure also passed the fire resistance test according to IEC 60331-1 (2009). After 90 minutes at 830° C. a signal attenuation was detected, but no optical fibers breakage were detected.

(32) In addition, Cable 2 according to the present disclosure was tested according to IEC 60332-3-24 (2000) and gave a flame propagation in vertical position of 770 mm (the maximum propagation according to the standard is 2,500 mm). Also Cable 2 was tested according to IEC 61034-2 (2005) relating to the density of the smoke generated by the cable while burning and gave a light transmission of 80.52% (the light transmission according to the standard has to be of at least 60%).